According to the state of the art the high-frequency signals received by the ultrasonic probes are digitized and can then be further processed by digital computers. The high frequency signals are also referred to as ultrasonic signals or as a so-called A-image. In the course of the further processing, a link can be made to the path coordinates of the measuring points, an evaluation and true-to-location representation of the test results (so-called B-, C- or D- images) can be performed, the measured results from several tests carried out in sequence on the same test object can be stored in order to determine changes, an analysis process can be carried out for the purposes of flaw reconstruction using known algorithms.
For most evaluation methods the signal travel time and/or the actual high-frequency signal waveform (the ultrasonic signal) is required in addition to the maximum signal amplitude in each flaw expectation range. For the digital processing the analogue signals must therefore be sampled at a multiple of the probe frequency in order to be able to sample the actual signal waveform with the required accuracy, i.e. in order to achieve the required accuracy for these evaluation methods. The relationship between probe frequency, sampling rate and sampling error is known: the higher the sampling rate selected in relation to the probe frequency, the lower is the sampling error.
If, for example, a probe signal with 15 MHz is to be digitized with a sampling error of 0.1 dB, then the sampling rate must be at 300 MHz. For an evaluation range of e.g. 100 mm (steel, longitudinal) this results for a single test shot in a volume of data of 10,240 digital measured values. If one assumes a testing density of one shot per millimeter of path, then this results in 10,240,000 measured values for a test path of 1 meter.
In practice, however, not only one but several, for example 2 to 16 and more test functions are provided for automatic tests and in addition high test speeds (e.g. 500 mm/s or greater) are necessary. On the basis of these standards up to 81,920,000 measured values per second would result for said example.
These amounts of data cannot be processed in real time with the economically feasible computers in practical use today and also cannot be stored.
In practice digitization is therefore only carried out at 100 MHz, thus a compromise is made between sampling error and the number of measured values arising. In addition the digitized data are reduced to maximum amplitude values per section of the travel path, e.g. 1 value per x mm. A reconstruction of the signal form to the degree of accuracy required for evaluation methods is however no longer possible with these values.